We are investigating the use of ultrathin (l.5 - l.8 mm) fiberoptic catheter systems for direct visualization of intraluminal anatomy in the coronary artery circulation. Using sheep which have been placed on total cardiopulmonary bypass, the aorta is cross clamped and the coronary ostia are cannulated and fine l.5 - l.8 mm angioscopes are passed into the right or left coronary arteries. The coronary arteries are perfusion fixed using a silver chloride to stain endothelium. The amount of endothelial denudation injury is assessed by scanning and optic microscopy. The coronary arteries of l0 sheep have been examined. The results show that major surface improvements to decrease endothelial injury are required. In addition, use of more powerful light sources to adequately visualize proximal coronary artery anatomy have been found necessary.